DISM /Online /Add-Package /PackagePath:"C:\Path\To\Your\File\Microsoft-Windows-NetFx3-OnDemand-Package.cab"
This article explains what this file does, where to find it, and how to install it properly. Microsoft-windows-netfx3-ondemand-package.cab Download
Replace D: with your actual mounted ISO drive letter. where to find it
If you are in an "air-gapped" environment (no internet) or dealing with a PC that has Windows Update restricted by group policy, you need the .cab file. Where to Download the .NET 3.5 CAB File Microsoft-windows-netfx3-ondemand-package.cab Download
(ISO or USB) Path inside the ISO: \sources\sxs\microsoft-windows-netfx3-ondemand-package.cab